1. Technical Field
The invention relates to telephone systems and, in particular, to so-called "number portability" whereby a subscriber may retain the same directory number when moving from one location and/or service provider to another or changing from one type of service to another, such as from wireless to landline or from business to residential.
2. Background Art
The invention is applicable to telephone systems which are known as "Intelligent Networks" (IN) or "Advanced Intelligent Networks" (AIN) and which employ out-of-band signalling systems such as Signalling System No. 7 (SS7), also known as Common Channel Signalling No. 7 (CCS7). Such signalling systems exchange Transaction Capabilities Application Part (TCAP) messages or queries between network elements to deploy selected services and ISDN-User Part (ISUP) messages between network node switching elements to set up and route calls. The TCAP and ISUP messages are handled by a data communications system separate from the trunks which carry the calls themselves. The elements of such intelligent telephone systems include Service Switching Points (SSPs) interconnected by trunks for carrying calls, and Signal Transfer Points (STPs) and Service Control Points (SCPs) connected to each other and the SSPs by SS7 data links for carrying signalling. A Service Control Point is an "intelligence center" with access to application databases enabling it to deliver various combinations of features, such as 1-800 number service and call redirection. A Signal Transfer Point (STP) is a signalling hub or concentrator, typically connecting several Service Switching Points to an SCP. A Service Switching Point (SSP) is a network node normally associated with a central office switch equipped with Signalling System Number 7 (SS7) messaging links to permit communication with the SCPs and which has special AIN call processing software including a plurality of "Point-in-Call triggers" which can be provisioned or set to interrupt call processing momentarily and initiate a TCAP query to the SCP for instructions on how to complete the call processing. Based upon the instructions, the originating end office seizes a trunk to a neighbouring switch and creates an Initial Address Message which it sends to the neighbouring switch via the signalling network. The Initial Address Message includes various parameters which will control routing of the call to its destination.
In circuit-switched telephone systems, each subscriber has a unique public directory number which is associated with a particular circuit of a switch at the subscriber's local office. As a general rule, if the subscriber transfers to a new service provider, the subscriber is assigned a new public directory number. For various reasons, it is desirable for a subscriber to be able to change service provider while keeping the same public directory number and any specific service features associated with it.
Various "local number portability" schemes have been proposed for "porting" a subscriber's directory number to a different switch and routing calls to it automatically, so that other subscribers may dial the same public directory number and still reach the subscriber, without knowing that the ported number is at a different switch. Procedures for achieving such "local number portability" were promulgated in a document entitled "Generic Switching and Signalling Requirements for Number Portability", Illinois Commerce Commission Workshop on LNP, Illinois, April 1996.
The former document proposes that, when a directory number is ported to a different switch, an entry will be made in the SCP database liking the ported directory number with the Location Routing Number of the recipient switch, i.e. the switch to which the subscriber transferred. Each originating end office will be notified that a number (NPA-NXX) from the "donor" office had been ported and will arm a trigger to query the SCP for instructions when trying to route any subsequent calls to that NPA-NXX. It is preferred to arm a trigger which will be actuated by any directory numbers beginning with that NPA-NXX rather than arm a trigger for each individual ported directory number. The SCP will return the Location Routing Number of the recipient switch. The originating switch will incorporate the Location Routing Number and the original dialled digits into a special Initial Address Message and route the call to the recipient switch, which will complete the call to the "ported" subscriber. The Initial Address Message is termed "special" because it will have the Location Routing Number in place of the usual "Called party number" and the original dialled digits (called number) as a parameter called the "Generic Address Parameter". This Generic Address Parameter is defined in Bellcore GR-697-CORE. At the recipient end office switch, the presence of the Location Routing Number (which cannot be dialled by a subscriber) in place of the called number will prompt the switch to look to the Generic Address Parameter for the called number to which the call should be completed.
Such an arrangement entails modifications to the SCP software, which is not particularly expensive because the SCPs are relatively few in number. A major disadvantage of this procedure, however, is that it requires every SSP in the network to be modified to give it the capability of creating the special Initial Address Messages and understanding those it receives. This entails considerable expense since there might be several hundred SSPs in the network.
Copending patent application Ser. No. 08/690,474 filed concurrently herewith, discloses and claims an improved method and apparatus for effecting number portability in a telephone network, particularly a so-called intelligent network, without requiring modification of the call processing software at originating network nodes or central office switches. In accordance with the invention of such copending application Ser. No. 08/690,474, there is provided a method of effecting directory number portability in a telephone network having a plurality of network nodes each having a unique network address and using out-of-band signalling for call setup and routing, such signalling system employing Initial Address Messages for controlling routing of the calls, such Initial Address Messages including selected ones of a Called Party Number, an Original Called Party Number, a Generic Address Parameter Number and a Forwarded Call Indicator, a method enabling routing of a call to a directory number that has been transferred from a donor switching unit at a donor network node in said telephone network to a recipient switching unit at a recipient network node in a different network controlled by a different service provider, the method including the steps of:
(i) in the telephone network, associating with a call to the ported directory number an Initial Address Message comprising the network address of the recipient network node as the Called Number and the ported directory number as the Original Called Number; PA1 (ii) at a boundary between said telephone network and said different network, converting the Initial Address Message by making the Original Called Party number a Generic Address Parameter and setting a Forwarded Call Indicator flag to indicate that the Called Number as originally-dialled has been translated; and PA1 (iii) routing the call into said different network using the converted Initial Address Message. PA1 (i) routing the call, using the ported directory number as the called party number, from an originating network node towards the donor network node; PA1 (ii) intercepting the call at an intermediate network node between the originating network node and the donor network node through which intermediate network node calls to the donor network node must pass,; PA1 (iii) converting the ported directory number/called party number to a network address of the recipient network node; PA1 (iv) routing the call to the recipient network node using said network address; and PA1 (v) at the recipient network node, completing the call to the ported directory number. PA1 (i) means for routing a call, using the ported directory number as the called party number, from an originating network node towards the donor network node; PA1 (ii) means for intercepting the call at an intermediate network node, between the originating network node and the donor network node, through which intermediate network node calls to the donor network node must pass,; PA1 (iii) means for converting the ported directory number/called party number to a network address of the recipient network node; PA1 (iv) means for routing the call to the recipient network node using said network address; and PA1 (v) means at the recipient network node for completing the call to the ported directory number.
Such copending patent application also deals with the situation where a ported directory number is ported to a recipient SSP within the telephone network, having been ported from a donor network node in the neighbouring or "different" network. In that case, calls to the ported number which originate in the other "different" network will have their Initial Address Messages converted at the boundary, but in reverse, i.e. the Forward Call Indicator flag will be dropped and the Generic Address Parameter digits will be transferred to the Original Called Party parameter. However, the copending application does not address the situation where a directory number is ported into the telephone company's network from the competing service provider's network and a caller within the telephone company's network tries to place a call to the ported directory number. In such a case, it is desirable for the telephone company to avoid routing the call into the service provider's network and having the service provider query its local number portability database and route the call back into the telephone company's network for completion. Not only could the service provider charge for accessing its database and returning the call back to the telephone company's network, but such a procedure would also cause what is known as "tromboning" by tying up two trunk circuits between the respective access nodes of the two networks.